The present invention relates to a method for the thermal treatment of bushings, particularly for tracked vehicles and the like.
Tracked vehicles are currently used for movement in particularly challenging and highly abrasive environments; accordingly, their tracks are subjected to considerable wear and even to fatigue caused by specific compressions and vibrations, and this requires rapid replacement in case of failure as well as a plurality of replacements during the working life of the tracked vehicle.
In particular, the track is composed of various components and substantially consists of an endless chain with which shoes are associated; said shoes are generally mutually bolted to a connecting assembly and can be disassembled independently of the other components of the track.
Said track is thus composed of links, pins and bushings in addition to gaskets, pusher rings and lubricators; these components are assembled by applying pressure to the respective ends of the connecting elements on the pins and on the bushings by means of high-power presses for tracks (which apply a force of approximately 50-60 tons).
Accordingly, if it is necessary to replace a specific worn component of the track, it is first of all necessary to remove the track from the vehicle and take said track to a workshop equipped with specific track presses, and this entails rather high costs and long times.
In detail, it must be also noted that each one of the various components of the track has an individual resistance to wear which can vary considerably among the components.
Originally, the problem of wear mainly involved the pins and the inner surface of the bushings coupled thereto; this problem has been overcome by using sealed and lubricated tracks which in fact have considerably reduced wear of the pin and of the corresponding internal seat of the bushing in which it is accommodated.
Among all the components, the one that is currently subject to the fastest wear is essentially constituted by the outer surface of the bushing; said wear occurs mainly because of the contact, and consequent friction, between the teeth of the driving sprocket when it engages and disengages said bushing.
The typical environment in which said tracked vehicles operate is also characterized by the presence of large amounts of abrasive materials, such as sand, slag, rocks, dirt and mud which thus make direct contact with the outer surface of the bushing, which in turn makes contact with the driving sprocket.
It should also be noted that the bushings are also subject to fatigue failures, which occur for example when the bushings are subjected to vibrations caused by excavation work or when they are subjected to intense compressions limited to specific points; it must be in fact considered that when for example excavators work on terrain which is uneven owing to the presence of rocks, holes, etcetera, a single bushing might have to bear the weight of the entire tracked vehicle.
Bushings can be obtained nowadays starting from a steel tube which is then machined or are normally produced starting from round steel bars by cutting, drilling or drawing with subsequent turning in order to make them assume the intended shape.
In order to extend the life of said bushings, they are conventionally subjected to thermal treatments which are essentially of two kinds: casehardening plus quench hardening, or core hardening and tempering plus internal and external induction hardening.
The purpose of these conventional treatments is to obtain a surface hardness which is sufficient to provide high wear resistance and toughness in the central part of the bushing (also known as "core") which is capable of withstanding fatigue stresses.
Surface hardness is measured in HRC; a surface hardness of more than 50 HRC is considered ideal for wear resistance, whilst a value of less than 45 HRC is considered necessary to ensure correct core toughness of the part.
Accordingly, a problem arising in the treatments of these bushings is the fact that it is necessary to try to bring the outer surface part to an optimum hardness down to a depth which is sufficient to ensure that the part is wear-resistant and at the same time to leave the core with a hardness which is low enough to ensure high toughness; however, the core must be hard enough to ensure resistance of the part to failures caused by intense specific pressures; moreover, in order to contain costs it would be necessary to leave the lowest possible high-hardness thickness for the inner surface of the bushing.
These problems are not solved in an optimum manner by the prior art, in which for example the casehardening plus quench hardening method is aimed at superficially enriching with carbon a steel which has a low carbon content (casehardening steel, UNI 7846/78 standards); the casehardening process occurs by raising the part to a temperature of approximately 950.degree. C. and making a gas with a high carbon content flow all around it inside a hermetic furnace (casehardening furnace).
The gas transfers part of its carbon content to the part by diffusion; for bushings, this process lasts approximately 20 to 30 hours of immersion in the gas, after which the part, once it has been cooled and removed from the casehardening furnace, is reheated to a temperature of approximately 900.degree. C. and drastically cooled with a quenching liquid, such as water, water with additives or other (quenching process).
Then the stress relieving step is performed; in other words, the part is heated, in an electric or gas or other furnace, to a temperature of 150-200.degree. C. for one or more hours, according to the size and type of treatment performed; this is done to increase the toughness of the casehardened region of the part and to eliminate the tensions inside the steel caused by said treatment.
A bushing treated in this way has, under micrographic observation, a metallurgical structure mainly of the martensitic type throughout its cross-section, and therefore with some residues of intermediate cooling structures (for example bainite) in the core in an amount directly proportional to the thickness of the part itself.
As a consequence of this conventional treatment, the part achieves high surface hardness, for example to a depth which can vary between 1.5 and 3.5 mm, and a lower hardness in the core; FIG. 2 illustrates a hardness curve of a casehardened bushing.
The conventional method described above, however, has the drawbacks mentioned earlier: it is in fact not possible, with this method, to differentiate the thickness of the harder layer between the outer surface and the inner surface of the bushing and the metallurgical structure in the core does not allow great fatigue strength.
Moreover, if it is intended to use this method to reach increasingly deeper layers, for example beyond 3.5 mm, with a hardness of more than 50 HRC, the casehardening temperature should be increased with the effect, however, that further problems regarding the life of the casehardening furnace itself would come out.
As an alternative, the part would have to be kept in the furnace for a longer time and this in any case would increase the treatment time and therefore the overall costs.
The ratio between the thickness of the hardened layer d and the casehardening time t is determined by the formula d=kt, where k is the constant determined by the type of steel, by the casehardening atmosphere and by the temperature; therefore, if thickness is to be increased for example from 4 mm to 7 mm the casehardening treating times have to be tripled, and this would entail a considerable increase in the costs of the operation; moreover, it has been observed that defects tend to form on the surface of the bushing in the form of an oxidized intergranular layer (abnormal casehardening layer), which in effect significantly reduces the fatigue strength of the component.
The conventional process for core hardening and tempering plus external and internal induction hardening is instead aimed at achieving mechanical and metallurgical characteristics which allow to improve wear resistance and fatigue strength with respect to the treatment by casehardening plus quench hardening; this conventional method, which provides for the use of a particular material to be processed, such as a hardened and tempered steel, essentially consists of four steps: the first step is also known as "mass hardening" and tempering and entails treatment by heating the part to a temperature above the austenitizing limit of the part (approximately 820-860.degree. C.) for a time which is sufficient for the complete conversion of the material (approximately 1 hour) and is then drastically cooled with a quenching liquid; the bushing is then heated further to a temperature which is below the critical transformation point and varies according to the intended hardness of the part at the end of the mass process; the tempering step lasts approximately 1 hour.
The second step of the process is also known as "external quench hardening": the part is superficially heated to a depth of approximately 3.5-6 mm with an electromagnetic induction system and cooled immediately with a jet of coolant liquid.
The third step is known as "internal quench hardening": the same process of heating followed by quick cooling is performed for the internal surface as well, with a hardening depth (depth of the hardened layer) of approximately 1-2.5 mm.
The fourth step is known as "stress relieving": the same type of stress relieving as in the casehardening plus quench hardening treatment is performed in order to increase the toughness of the part and eliminate the tensions inside the steel caused by the treatment itself.
Graphically, the result can be shown by FIG. 3, wherein the metallurgical structure obtained with this conventional method has, at the core, a tempered martensitic structure (sorbite) whilst there is provided a martensitic structure on the surface regions of the inside and outside diameters of the bushing (not on the heads).
Two regions having a mixed structure (ferrite, bainite, pearlite) form between the outer regions and the core of the bushing: the presence of a sorbitic structure in the core allows to have a much higher toughness than obtained with a martensitic structure (as in the casehardening plus quench hardening process).
FIG. 4 plots the hardness of the part obtained in this manner: it shows that it is possible to obtain a thickness with a hardness of more than 50 HRC which is greater on the outside diameter and smaller on the inside diameter, with thickness values which can be much higher than the 3.5 mm that can be obtained with the casehardening process, whilst the core decreases to a hardness of 20-25 HRC.